Method for making a vertical-cavity surface emitting laser with improved current confinement

ABSTRACT

A current confinement element that can be used in constructing light-emitting devices. The current confinement element includes a top layer and an aperture-defining layer. The top layer includes a top semiconducting material of a first conductivity type that is transparent to light. The aperture-defining layer includes an aperture region and a confinement region. The aperture region includes an aperture semiconducting material of the first conductivity type that is transparent to light. The confinement region surrounds the aperture region and includes a material that has been doped to provide a high resistance to the flow of current. The aperture-defining layer is constructed by implanting or diffusing elements into one or more of the mirror layers prior to depositing the remaining mirror layers on top of the aperture-defining layer.

FIELD OF THE INVENTION

This invention relates generally to vertical cavity surface-emittinglasers (VCSELs) and, more particularly, to an improved VCSEL in whichthe current channeling function utilizes an ion-implanted or diffusedaperture region.

BACKGROUND OF THE INVENTION

Semiconductor laser diodes were originally fabricated in a manner thatled to a diode structure in which light is emitted parallel to thesurface of the semiconductor wafer with a cavity constructed frommirrors that are perpendicular to the surface of the substrate.Unfortunately, this structure does not lend itself to low cost “mass”manufacturing or to the cost-effective fabrication of two-dimensionalarrays of laser diodes.

These problems are overcome by a class of laser diodes that isfabricated such that the laser structure is perpendicular to the surfaceof the semiconductor wafer and the light is emitted perpendicular to thesurface. These laser diodes are commonly known as Vertical CavitySurface-Emitting Lasers (VCSELs). A VCSEL may be viewed as a laserhaving mirrors constructed from alternating layers of material havingdifferent indices of refraction. These lasers are better suited for thefabrication of arrays of lasers for displays, light sources, opticalscanners, and optical fiber data links. Such lasers are useful inoptical communication systems for generating the light signals carriedby optical fibers and the like.

Compared to conventional edge-emitting semiconductor lasers, VCSELs havea number of desirable characteristics. The use of multi-layered DBRmirrors to form a cavity resonator perpendicular to the layerseliminates the need for the cleaving operation commonly used to createthe cavity mirrors used in edge emitting lasers. The orientation of theresonator also facilitates the wafer-level testing of individual lasersand the fabrication of laser arrays.

To achieve high-speed operation and high fiber-coupling efficiency, itis necessary to confine the current flowing vertically in the VCSEL, andthus the light emission, to a small area. There are two basic prior artcurrent confinement schemes for VCSELs. In the first scheme, aconductive aperture is defined by means of an ion-implanted,high-resistivity region in the semiconductor Distributed-Bragg-Reflector(DBR) mirror. Such a scheme is taught in Y. H. Lee, et al., Electr.Lett. Vol. 26, No. 11, pp. 710-711 (1990), which is hereby incorporatedherein by reference. In this design, small ions (e.g. protons) aredeeply implanted (e.g. 2.5 to 3 μm) in the DBR mirror. The implantationdamage converts the semiconductor material through which the ionstraveled to highly resistive material. Current is provided to the lightgeneration region via an electrode that is deposited on the top surfaceof the VCSEL. To facilitate a low-resistance electrical contact throughwhich light can also exit, an annular metal contact is deposited on thetop-side of the device. The contact typically has an inner diametersmaller (e.g. by 4 to 5 μm) than the ion-implantation aperture in orderto make contact with the non-implanted conducting area. As a result, aportion of the current-confined, light-emitting area is shadowed by theannular metal contact. The percentage of the current-confined,light-emitting area shadowed by the annular metal contact increasesdrastically as the size or diameter of the current-confined,light-emitting area decreases. This factor limits the smallest practicalsize of the current-confined area, and, therefore, limits the speed andlight-output efficiency of this type of VCSEL.

In addition, ion-implanted VCSELs exhibit multiple spatial modes, whichlead to a light-output-versus-current curve that often displays kinksdue to the random and varying nature of the multiple spatial modes. Thiskinky light-output-versus-current behavior can produce a “noisy”waveform, and can degrade the performance of optical communicationsystems based on such designs. Prior art devices have been proposed toreduce the kinks in the light-output-versus-current characteristics byintroducing additional structures in the light-emitting area; however,such solutions increase the cost and complexity of the devices.

In the second design, the current confinement aperture is achieved bygenerating a high-resistivity oxide layer embedded in the semiconductorDBR mirror. Such a scheme is taught in D. L. Huffaker, et al., Appl.Phys. Lett., vol. 65, No. 1, pp. 97-99 (1994) and in K. D. Choquette, etal., Electr. Lett., Vol. 30, No. 24, pp.2043-2044 (1994), both of whichare incorporated herein by reference. In this design, an annularinsulating ring having a conducting center is generated by oxidizing oneor more layers of Al-containing material in the DBR mirror in ahigh-temperature wet Nitrogen atmosphere. The size of the oxide apertureis determined by the Al concentration of the Al-containing layers, thetemperature, the moisture concentration of the oxidizing ambient, andthe length of the oxidation time. The oxidation rate is very sensitiveto all of these parameters, and hence, the oxidation process is not veryreproducible. As a result, device yields are less than ideal. Inaddition, the oxidization process leads to stress within the device,which can further reduce yields.

Broadly, it is the object of the present invention to provide animproved VCSEL and method for making the same.

The manner in which the present invention achieves its advantages can bemore easily understood with reference to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings.

SUMMARY OF THE INVENTION

Broadly, the present invention is a current confinement element that canbe used in constructing light emitting devices. The current confinementelement includes a top layer and an aperture-defining layer. The toplayer includes a top semiconducting material of a first conductivitytype that is transparent to light. The aperture-defining layer includesan aperture region and a confinement region. The aperture regionincludes an aperture semiconducting material of the first conductivitytype that is transparent to light. The confinement region surrounds theaperture region and includes a material that has been doped to provide ahigh resistance to the flow of current. In one embodiment of the presentinvention, the confinement region includes a confinement semiconductingmaterial of a second conductivity type. In another embodiment of thepresent invention the confinement region includes a material that hasbeen doped with impurities that increase the resistivity of the materialto a value greater than 5×10⁶ ohm-cm. The aperture-defining layer is inelectrical contact with the top layer such that current will flowpreferentially through the aperture region relative to the confinementregion when a potential difference is applied between the top andaperture defining layers.

The present invention can be used as part of a laser diode by utilizingpart of one of the mirrors as the aperture-defining layer. A laseraccording to the present invention is fabricated by growing the layersin the conventional manner through the light emitting layer and firstspacer layer. The first few layers of the top mirror are then grown. Thedevice is then removed from the growth chamber, and the aperture regionis masked. The unmasked area is then implanted or diffused withimpurities to create the confinement region. The mask is then removed,and the device is returned to the growth chamber where the remainingmirror layers are fabricated in the conventional manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional ion-implanted VCSEL10.

FIG. 2 is a cross-sectional view of a VCSEL 100 according to onepreferred embodiment of the present invention.

FIGS. 3(A)-3(E) are cross-sectional views of a VCSEL according to thepresent invention at various stages in the fabrication process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be more easily understood with reference toFIG. 1, which is a cross-sectional view of a conventional VCSEL 10.Since construction of VCSELs is well known to those skilled in the laserarts, it will not be described in detail here. For the purposes of thisdiscussion, it is sufficient to note that VCSEL 10 may be viewed as ap-i-n diode having a top mirror region 18, a light generation region 14,and bottom mirror region 19. These regions are constructed on asubstrate 12. Electrical power is applied between electrodes 22 and 27.The various layers are constructed by epitaxial growth. Substrate 12 isan n-type semiconductor in the example shown in FIG. 1.

The active region is typically constructed from a layer having one ormore quantum wells of InGaAs, GaAs, AlGaAs, InGaAsN, or InAlGaAs that isseparated from mirror regions 18 and 19 by spacers 15 and 16,respectively. The choice of material depends on the desired wavelengthof the light emitted by the VCSEL. In addition, devices based on bulkactive regions are known to the art. This layer 14 may be viewed as alight generation layer which generates light due to spontaneous andstimulated emission via the recombination of electrons and holesgenerated by forward biasing the p-i-n diode.

The mirror regions are constructed from alternating layers of whichlayers 20 and 21 are typical. These layers have different indices ofrefraction. The thickness of each layer is chosen to be one quarter ofthe wavelength of the light that is to be output by the VCSEL. Thestacked layers form Bragg mirrors. The stacks are typically constructedfrom alternating layers of AlAs or AlGaAs and GaAs or AlGaAs of lower Alconcentration. The layers in the upper mirror region 18 are typicallydoped to be p-type semiconductors and those in the lower mirror region19 are doped to be n-type semiconductors. Substrate 12 is preferably ann-type contact. Bottom electrode 27 is preferably an n-ohmic contact.However, n-i-p diode structures may also be constructed by growing thestructures on a p-substrate or a semi-insulating substrate with ap-layer deposited thereon.

In one of the prior art designs discussed above, the current flowbetween electrodes 22 and 27 is confined to region 24 by implantingregions 25 and 26 to convert the regions to regions of high resistivity.This is typically accomplished by implanting with hydrogen ions. Toprovide sufficient current through the light generating region,electrode 22 must extend over the light generating region to somedegree. As noted above, this overlap can cause a significant reductionin device efficiency in small VCSELs.

Refer now to FIG. 2, which is a cross-sectional view of a VCSEL 100according to one preferred embodiment of the present invention. VCSEL100 utilizes a buried current confinement structure, which enables VCSEL100 to have a higher output light efficiency and is better suited toVCSELs having a small emission area. VCSEL 100 is similar to VCSEL 10discussed above in that VCSEL 100 has a bottom mirror region 110, alight generation region 111, and a top mirror region 112. The mirrorregions are Bragg reflectors and are constructed by epitaxial growth ofthe alternating layers described above. VCSEL 100 is also a p-i-n diodethat is forward biased by applying a potential between electrodes 121and 122. The bottom mirror layers are preferably grown on a buffer layer135 of the same conductivity type as the bottom mirror layers. Theactive region 111 includes a spacer layer 136 of the same conductivitytype as the bottom mirror layers. The active region also includes alight emitting layer 137, and a second spacer having the oppositeconductivity type, i.e., the conductivity of the type of materials usedfor the upper mirror 112.

Current confinement is provided by a buried annular region 131 thatrestricts current flow to the center 132 of the annulus. The manner inwhich this current confinement region is constructed will be discussedin detail below. It should be noted that the buried nature of theconfinement structure leaves the region 133 between electrode 121 andregion 131 in a conducting state. Accordingly, region 133 can provide acurrent spreading function, and hence, electrode 121 does not need tooverlap the current conducting portion of region 131.

In the preferred embodiment of the present invention the reflectivity ofthe mirror layers in region 131 is less than that of the mirror layersin region 132. The resultant VCSEL has been found to exhibit a smootherlight output versus current curve.

In the preferred embodiment of the present invention, thecurrent-channeling structure is formed by ion-implantation or diffusionof species into one or more of the layers that make up the top mirror.Refer now to FIGS. 3(A)-3(E), which are cross-sectional views of a VCSELaccording to the present invention at various stages in the fabricationprocess. The various layers needed to provide the bottom mirror 210,active region 211, the first few layers of the top mirror 212 and a thinGaAs cap layer 234 are first fabricated in the conventional manner asshown in FIG. 3(A). The GaAs cap layer is in the order of 50-100 Å thickand is positioned at one of the nulls of the optical field in theoptical cavity. This arrangement minimizes the absorption of the lightby this GaAs layer while protecting the underlying layers of thestructure from oxidation.

The wafer on which the layers have been deposited is then removed fromthe epitaxial growth chamber. A mask 213 is then generated over theregion that is to become the aperture of the current-channelingstructure. The mask is preferably generated by conventional lithographictechniques. The wafer is then implanted or diffused with atomic speciesto provide the current-confinement regions 214 as shown in FIG. 3(B).The particular species utilized will be discussed in more detail below.

After the introduction of the doping species, the mask is removed andthe remaining layers 215 of the top mirror are deposited as shown inFIG. 3(C). The implanted or diffused area must be preserved during thehigh temperature exposure of the altered region that is encounteredduring the growth of the remaining mirror layers.

After the deposition of the remaining layers 215 of the top mirror, ahigh resistive region 217 as shown in FIG. 3(D) may be created by anaggregated implantation to reduce the parasitic capacitance of the VCSELunder its top metal contact and bonding pad. If the individual VCSELsare part of an array in which the members are not isolated by cutting atrench between the members of the array, this aggregated implantationshould be extended to beyond the light emitting layer to provideisolation of the individual VCSELs as shown at 227 in FIG. 3(E). Theisolation implant is constructed by masking the surface of the topmirror and then implanting the unmasked region with hydrogen ions torender the region non-conducting, or at least highly resistive. Sincethe isolation implant does not define the current confinement region inthe device, the isolation implant does not need to extend over theaperture in the current-channeling structure. Hence, the problemsdiscussed above with respect to such implants are not encountered in thepresent invention.

Finally, the top contact, which includes electrode 218, is deposited.This contact may include a grading layer and contact layers of the sameconductivity type as the upper mirrors in addition to the metallicelectrode. It should be noted that the top electrode does not need toextend over the region defined by the aperture in the current-channelingstructure, since the mirror layers below the electrode are electricallyconducting, and hence provide a current spreading function. The topelectrode only needs to extend beyond the current of the optionalisolation implant discussed above.

The specific implants or diffusion species will now be discussed in moredetail. To simplify the following discussion, the term “implant” shallbe deemed to include the introduction of a species either by bombardmentat an appropriate energy or by diffusion of the species into thematerial. The implanted region must present a very high resistance pathcompared to the material in the aperture of the current-channelingstructure. One method for accomplishing this goal is to alter theresistivity of the material outside of the aperture. Examples of implantspecies for generating such a high-resistivity region are O, Ti, Cr, orFe; other species will be apparent to those skilled in the art. In thepreferred embodiment of the present invention, the high-resistivitylayer has a resistivity greater than 5×10⁶ ohm-cm.

A second method for generating a high resistance current path is toimplant the region with species that change the conductivity-type to theopposite conductivity, thereby generating a reversed-biased diodeoutside of the current-defining aperture. For example, if the region tobe implanted is initially a p-type material, the material in the implantregion can be converted to an n-type material by implanting the regionwith Si, Ge, S, Sn, Te, Se or other species that render the regionn-type. If the material in the implant region is initially an n-typematerial, C or Be may be implanted to render the region p-type.

It should be noted that the aperture of the current-channeling structuremay also be formed by ion-implantation or diffusion of a combination ofspecies which form a region of high-resistivity and/or a region ofopposite-type conductivity from the original material. In all cases,either or both of the high-resistivity region or the region ofopposite-type of conductivity, may be created using a plural number ofimplant energies and/or dosages. It has been found experimentally thatthe characteristics of high-resistivity or opposite-type of conductivityprovided by the above-described implants are preserved through the hightemperature processes that are required to grow the remaining portion ofthe top DBR mirror.

The embodiments of the present invention discussed above utilize acircular annulus for the current-defining aperture. However, it will beobvious to those skilled in the art from the preceding discussion thatother shapes may be utilized for the current confinement aperture. Incontrast to oxide VCSELs, the shape of the aperture in the presentinvention is determined by a lithographic mask, and hence, the designeris free to chose any shape aperture. Since the aperture determines thecross-section of the resultant light signal, the present inventionprovides additional advantages over oxide VCSELs.

The embodiments of the present invention described above have referredto top and bottom mirrors, etc. However, it will be obvious to thoseskilled in the art from the preceding discussion that these terms areconvenient labels and do not imply any particular spatial orientation.

The energy and dosage of implantation depends on the species used andthe concentration of the dopants in the first few pairs of layers of thetop DBR mirror. As an example, for a typical doping concentration of10¹⁸ cm⁻³ of the top DBR mirror, an implant energy of 300 KeV and doseof about 5×10¹⁴ cm⁻² may be employed for Si. Different implantationenergies may be used for other atom species to place the implantedregion at the desirable depth. Si may also be introduced by solid statediffusion. The diffusion temperature and time are chosen to provide thedesirable depth. For example, diffusion at a temperature above 700° C.for a few hours can be utilized for Si.

In principle, the current-channeling structure can be constructed ineither mirror; however, in the preferred embodiment of the presentinvention, the structure is formed in the p-type DBR mirror because itprovides better current confinement. If the current-channeling structureis on the n-side, current tends to spread out between thecurrent-channeling structure and the active region, due to the higherconductivity of an n-type AlGaAs semiconductor. In addition, it isadvantageous to grow the active light-emitting layer on as high aquality semiconductor surface as possible. If the current-channelingstructure is constructed in the n-type mirrors prior to the depositionof the active region, the active region must be grown on a surface thathas been subjected to masking, implantation, etc. These processes will,in general, reduce the quality of the semiconductor surface, and hence,much greater care must be taken in fabricating the current-channelingstructure if it is to be under the active layer.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

What is claimed is:
 1. A method for fabricating a light-emitting device,said method comprising the steps of: depositing a first semiconductinglayer of a first conductivity type; depositing a light-emitting layer onsaid first semiconducting layer; depositing a second semiconductinglayer of a second conductivity type on said light-emitting layer;depositing an aperture defining layer on said second semiconductinglayer, said aperture defining layer comprising a semiconducting materialof said second conductivity type; defining an aperture region in saidaperture defining layer; implanting an area around said aperture regionwith impurities to create a current-blocking region, said apertureregion remaining free of said impurities; and depositing a thirdsemiconducting layer of said second conductivity type on said secondsemiconducting layer after said implanting step.
 2. The method of claim1 wherein said impurities convert said semiconducting material of saidaperture defining layer to said first conductivity type.
 3. The methodof claim 2 wherein said impurities comprise an element chosen from thegroup consisting of C, Be, Si, Ge, S, Sn, Te, and Se.
 4. The method ofclaim 1 wherein said impurities convert said semiconducting material ofsaid aperture defining layer to a material having a resistivity greaterthan 5×10⁶ ohm-cm.
 5. The method of claim 4 wherein said impuritiescomprise an element chosen from the group consisting of O, Cr, Ti, andFe.
 6. The method of claim 1 wherein said impurities comprise an elementchosen from the group consisting of O, Cr, Ti, and Fe.
 7. The method ofclaim 1 wherein said first semiconducting layer comprises a plurality ofsub-layers forming a DBR mirror.
 8. A method for fabricating alight-emitting device, said method comprising the steps of: depositing afirst semiconducting layer of a first conductivity type; depositing alight-emitting layer on said first semiconducting layer; depositing asecond semiconducting layer of a second conductivity type on saidlight-emitting layer; depositing an aperture defining layer on saidsecond semiconducting layer, said aperture defining layer comprising asemiconducting material of said second conductivity type; defining anaperture region in said aperture defining layer; implanting an areaaround said aperture region with impurities to create a current-blockingregion, said aperture region remaining free of said impurities; anddepositing a third semiconducting layer of said second conductivity typeon said second semiconducting layer after said implanting step, whereinsaid aperture defining layer comprises a plurality of sub-layers forminga portion of a DBR mirror, said third semiconducting layer comprisinganother portion of said DBR mirror.